NMR studies of superconductingNa2AC60(A=Cs, Rb, and K) fullerides

Multiple-nuclei NMR investigations on fullerides that contain sodium, ${\mathrm{Na}}_2$A${\mathrm{C}}_{60}$ (A=K,Rb,Cs), with a simple-cubic structure are reported. The density of states at the Fermi level vs lattice constant correlation was determined. The superconductivity in ${\mathrm{Na}}_2$A${\mathrm{C}}_{60}$ connot be described by the same parameters as those of the other ${\mathit{A}}_3$ ${\mathrm{C}}_{60}$ superconductors within the Bardeen-Cooper-Schrieffer theory. We find that structural imperfection plays an important role in determining the electronic states and onset of superconductivity in ${\mathrm{Na}}_2$A${\mathrm{C}}_{60}$. Evidence for the orientational ordering transition was also found with ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ NMR for A=Cs, Rb, and K.